The distribution of galaxies provides a powerful way to probe the properties of our universe. In order to
exploit this observable properly it is necessary to understand what we are really measuring when we look at
the large-scale structure. Since our universe is not completely homogeneous and isotropic, we only see a
distorted picture of our sky. In this talk, I will discuss the various relativistic effects that distort our
observations. I will show that even though these effects complicate the interpretation of galaxy surveys, they
are very useful since they contain information about the theory of gravity and can therefore be regarded as a
new opportunity for future surveys.